The present invention relates to a device for the hydraulic boosting of the steering force in an automobile. The device typically comprises a hydraulic cylinder whose working piston is coupled to the steering rod assembly. Pressure fluid is introduced from an auxiliary hydraulic energy source via multi-directional control valves into the working chambers of the device.
In one known steering unit of this type, disclosed in German Patent Publication 30 06 297, multi-directional control valves are electrically controlled through rapid break switches or angular switches when the steering wheel is turned. As a result, hydraulic pressure is supplied into the right or the left working chamber of the hydraulic piston, depending on the angle of steering turn desired, with the pressure boosting the steering actuating force. To conserve the auxiliary energy required, this unit arranges for the associated pump to be started via the electric driving motor only when the steering wheel is actuated. The unit accordingly requires undue component parts and is expensive to manufacture.
In another known servo steering unit described in German Patent Publication 32 22 631, the electric driving motor of the auxiliary energy supply system is connected to the oil pump via an electromagentic clutch. The driving efficiency of the oil pump is permitted to conform to system requirements irrespective of the engine's revolutions or the vehicle speed. Again, such an auxiliary energy supply system is comparatively expensive to manufacture.
It is also known from German Patent Publication 32 22 631 to conform the power consumption of a steering aid unit to the conditions prevailing at very low speed and when the vehicle is at standstill. The steering aid is reduced with increasing speed to augment driving stability.
Accordingly, it is an object of the present invention to provide an improved, effective and reliable steering aid unit.